It is known to use quadrupole rod sets to filter ions according to their mass to charge ratio. Different combinations of RF and DC voltages may be used to select the mass to charge ratios that are transmitted by the quadrupole. The RF and DC voltages are typically fixed for a first period such that the quadrupole selectively transmits only ions having a first mass to charge ratio of interest. The RF and DC voltages are then stepped such that in a second period the quadrupole selectively transmits only ions having a second mass to charge ratio of interest. Such methods may be used, for example, to select ions in single ion recording (SIR), single reaction monitoring (SRM) and multiple reaction monitoring (MRM) experiments.
When a quadrupole is used in this manner the ion current that is transmitted during the first period may be very large, whereas the ion current transmitted during the second period may be relatively small. The first, large ion current can cause the detector baseline to shift. For example, if a photomultiplier is used as the detector, a large ion signal can cause the photo-cathode of the detector to become excited and emit electrons for a significant period of time after the stimulus has been removed. Such baseline shifts can cause measurement errors for the channel(s) that follow the high intensity channel.
It is known to measure the detector's baseline level prior to an analytical acquisition. The baseline level can then be subtracted from ion signals measured during the analytical run. However, such methods are unable to take into account shifts in the baseline level that can occur after a high ion current has been detected.
Many quadrupole voltage driving circuit designs cause the DC voltage component to lag the RF voltage component. When the quadrupole is stepped so that the mass to charge ratios of the ions that are transmitted increases with time, the DC voltage component is temporarily lower than the RF voltage component . This temporarily allows ions having a wide range of mass to charge ratios to be transmitted by the quadrupole. Other voltage driving circuit designs cause the DC voltage component to lead the RF voltage component. When the quadrupole is stepped so that the mass to charge ratios of the ions that are transmitted decreases with time, the quadrupole may de-resolve. Again, this results in a relatively large pulse of ions being temporarily transmitted by the quadrupole. The amplitude of the ion pulse depends upon the number of ion species near to the analytes being measured and to their abundancy. It will therefore be appreciated that the stepped operation of a quadrupole may sometimes result in relatively large pulses of ions impinging on the downstream devices, such as an analytical mass filter or detector, each time that the quadrupole is stepped. If relatively large pulses of ions arrive at such a downstream device it can have deleterious results.
Mass spectrometers employing quadrupole mass filters typically gather data only when the quadrupole filtering action is at steady state, i.e. when the RF:DC ratio is substantially fixed. For example, if analytes A and B are to be analysed, the system will change the required RF and DC voltage components so as to filter all ions except those having a mass to charge ratio corresponding to that of analyte A, will then wait for the voltages on the electrodes of the quadrupole to settle so as to facilitate suitable mass resolution, and will then measure and record the ion current for a period of time. The system will then stop recording the ion current before programming the next RF and DC values for analyte B, and will wait for the voltages on the electrodes of the quadrupole to settle prior to recording the ion current for analyte B. The ion currents are then stored in separate channels so as to allow for further data processing. Consequently, the ion current is not recorded or displayed whilst the RF and DC voltages are unstable (i.e. between step values) as this data is not analytically useful.
Thus the deleterious nature of the ion pulses caused by temporary quadrupole de-resolution goes unseen. However, their potential effect on data quality is real, causing shifts in the detector baseline which extend into the scan or dwell period where ions currents are measured and hence may cause mis-quantitation of analytes.
It is desired to provide an improved mass spectrometer and an improved method of mass spectrometry.